The long-term objective of this proposal is to understand the cause of macular degeneration. A mutation (R345W) in fibulin-3 (EFEMP1) causes Malattia Leventinese (ML, also known as Doyne's honeycomb retinal dystrophy), an inherited macular degenerative disease with strong similarities to age-related macular degeneration (AMD). Fibulin-3 has been found to aberrantly accumulate beneath the retinal pigment epithelium (RPE) in both ML and AMD and strongly interact with TIMP-3 which is mutated in Sorsby's fundus dystrophy (SFD), another inherited macular degenerative disease. ML, SFD, and AMD are characterized by sub-RPE deposits and share close similarities in symptoms and histopathology. Fibulin-3 belongs to the fibulin family of six known extracellular matrix (ECM) proteins. Recently missense variations in other fibulins have been detected in AMD patients. These data strongly implicate fibulins in a general pathogenic pathway leading to macular degeneration. The functions of fubulins and their mode of involvement in disease are poorly understood. Fibulin-3 has highest homology to fibulin-4 and 5. In preliminary studies, we found that mice lacking fibulin-3 exhibit elastic fiber defects in skin and Bruch's membrane. In contrast, mice carrying the R345W mutation have a severely altered elastic fiber layer and basal linear deposit-like accumulation of materials in Bruch's membrane. Mice lacking fibulin-4 do not form elastic fibers. It has been shown that mice lacking fibulin-5 have disrupted elastic fibers. These findings reveal critical functions of fibulins in elastic fiber assembly and the importance of the elastic layer of Bruch's membrane in macular degeneration. The hypothesis of this proposal is that mutated fibulin-3 alters elastic fiber assembly resulting in its accumulation and changes in Bruch's membrane's structure and function. Bruch's membrane provides a semipermeable filtration barrier for bidirectional diffusion of nutrients and metabolites between the outer retina and the choriocapillaris. Guided by this hypothesis, specific aims are proposed to determine the precise role of fibulin-3 in elastic fiber assembly, how the R345W mutation alters or compromises this function, and how these alterations affect Bruch's membrane's function. These studies will provide fundamental new insights into how changes in Bruch's membrane lead to macular degeneration.